The present invention relates to the measurement of the pressures of fluids, particularly gases, with very high precision.
It has as aim to provide an apparatus which allows the very accurate measurement to be made not only of an absolute pressure but also of a relative pressure (with respect to the atmospheric pressure), or even a difference of pressures (difference between the pressures of two fluids).
The apparatus of the invention may in particular form a pressure standard for checking and calibrating pressure gauges, or a pressure sensor or transmitter.
In French Pat. No. 2 481 801 filed by the applicant on the Apr. 30, 1980 a device was described for the very accurate measurement of fluid pressures with a display of the measured pressure, characterized by the fact that it comprises, in combination, a vertical cylinder, a piston adapted for sliding in this cylinder with viscous friction, the pressure to be measured being applied to the upper face of the piston, means for rotating said piston in said cylinder, a precision electromagnetic dynamometric display balance or scale comprising a shaft to which the force to be measured is applied and means for transmitting to said shaft of the balance the force applied to the upper face of the piston by the pressure to be measured, with limitation of the maximum forces.
The device of this prior French patent in fact measured a relative pressure, namely that of a fluid applied to the upper face of said piston, with respect to the atmospheric pressure which acted on the lower face of this piston.
Another device for measuring a relative pressure is described in the article published in the review Control and Instrumentation, vol. 11, No. 5 (London, May 1979), pages 29 and 31. This device, which is manufactured by one of the applicants, namely the firm DESGRANGES ET HUOT (whose initials are D & H), comprises means for rotating the piston in the measurement cylinder while providing good lubrication in the clearance between the rotating piston and the fixed cylinder by providing for example (FIG. 2B page 31 of this article) a receptacle connected by piping, on the one hand, at its lower part to said clearance and, on the other hand, at its upper part to a chamber disposed under the piston and connected to the pressure to be measured, said receptacle being partly filled with lubricant as well as the piping which connects the lower part of the receptacle to the clearance. In this device, the lower face of the piston which plunges in said chamber is subjected to the pressure to be measured, whereas the upper face of the piston is subjected to the ambient atmospheric pressure, which allows a differential pressure measurement to be made.
Furthermore, the European application No. 0 112 647 published on the July 4, 1984 describes a pressure measuring device comprising a piston to one end of which is applied the pressure to be measured and whose other end is applied against the mobile part of a precision balance, said piston being movable longitudinally in a sleeve which is rotated, the piston itself being secured against rotation. Such a device is in fact a variant of the device described in the above mentioned French Pat. No. 2 481 801, rotation of the piston in this cylinder provided for in this patent No. 2 481 801 being replaced by the rotation of the sleeve (serving as cylinder) about the piston in the European application No. 0112 647.
In these three prior documents (French Pat. No. 2 481 801, the article Control and Instrumentation and the European application No. 0112 647) the piston receives on one face the pressure to be measured whereas the other face of the piston acts on the mobile part of a precision balance (in the above cited patent and patent application), or else receives calibrated weights (above cited article). Thus a relative pressure measurement is made with respect to the atmospheric pressure.
Contrary to this prior teaching, the present invention aims at providing a standard of measurement of a fluid pressure, particularly the absolute pressure of a gas, by providing application of the pressure to be measured to the upper face of a pressure measuring piston, rotating in a cylinder, while applying to the lower face of the pressure measuring piston, depending on the desired application, either a vacuum (for carrying out an absolute pressure measurement), or the atmospheric pressure (for carrying out a relative pressure measurement, with respect to this atmospheric pressure), or another desired pressure (for carrying out a pressure difference measurement).
For this, the invention provides a device for the very accurate measurement of fluid pressures with a display of the measured pressures, comprising a vertical cylinder, a piston adapted for sliding in this cylinder with viscous friction, the pressure to be measured being applied to the upper face of the piston in an upper enclosure, means for rotating said piston in said cylinder, a precision electromagnetic dynamometric display balance comprising a piece to which the force to be measured is applied and means for transmitting to said piece of the balance the force applied to the piston by the pressure to be measured, with limitation of the maximum forces, which device is characterized in that it comprises, in combination with the cylinder-piston assembly; a lower sealed enclosure at the lower part of the cylinder-piston assembly, this lower enclosure being able to be evacuated or pressurized to a desired pressure (the atmospheric pressure or that of a fluid); and means for permanently supplying with fluid the clearance between the piston and the cylinder so as to ensure correct centering of the piston in the cylinder, even if said upper and lower enclosures are evacuated.
Preferably, said means are formed by an annular groove provided more especially at mid height of the cylinder and fed with clean and dry gas at the atmospheric pressure.
The device further comprises advantageously means for measuring the temperature at the level of a cylinderpiston assembly and means for correcting the value of the measured pressure as a function of its temperature.